When is the extension of stay due for an O-A visa issued in London if I entered Thailand on April 4th, 2021?

You're confusing two separate things entirely

ONE is the visa (sticker) validity and that's a year from the date it's issued (meaning you can enter/exit thailand as many times as you want during the year the visa is valid for.

BUT

A visa doesn't let you stay here, it just lets you in with the amount of time allowed by that particular visa type. What lets you stay IN thailand is the entry stamp you got when you stamped in. Now on an O-A visa it gets you stamped in for a year from the date you entered (as long as you're insured for that amount of time)

So if you're not going to exit/re-enter the country just before the VISA (the sticker) expires to get another year entry stamp your FIRST yearly extension at the immigration office should be coming due around April 3rd 2020

Most offices let you apply for a new extension when you have 30 days or less left on your entry stamp, so you could apply for your year extension any time after March 3rd 2022

Look in your passport for your "admitted to" stamp. It should be stamped one year after you entered Thailand (so around April 3, 2022).

You will apply for an extension of stay based on retirement AT MOST IMMIGRATION OFFICES when you have 30 days or less remaining on your existing stamp.

You have a Non-OA visa, which is multiple entry. IF the borders reopen, I highly recommend bouncing out of Thailand in late March/ early April, for another free one year stamp. Then you won't have to get an extension of stay until early 2023

LOOK at the entry stamp you go when you stamped in to the country , if you stamped in April 4th and got an entry stamp saying "admitted until" April 3, 2022 that's when your extension would be due